This invention is directed to a charge control circuit for regulating the application of charging current to a secondary battery utilized as a power supply in a small-sized electronic instrument, and in particular to a charge control circuit that detects the voltage level of the battery being charged, and, in response thereto, selectively controls the charging current applied thereto.
In conventional small-size portable electronic instruments, such as electronic wristwatches, pocket calculators and the like, a small-sized, low capacity DC battery is utilized as a power source. For example, mercury oxide or silver oxide batteries having a useful life of one to two years, are utilized as a power supply, the useful life determining the frequency with which the battery must be replaced.
In order to eliminate the inconvenience of having to change batteries in such electronic instruments, secondary batteries have been utilized as a power supply. Although efforts have been made to include solar batteries as charging devices in electronic instruments, the use of secondary batteries for storing the charging voltage, supplied by the solar battery, have produced an unsatisfactory result, due in large measure to the inefficiency of the secondary battery.
Specifically, button shaped Ni-Cd batteries have been utilized as secondary batteries in electronic wristwatches. It is noted, however, that the voltage level of Ni-Cd batteries is often as low as 1.2 V, which is small in comparison with the 1.3 V and 1.5 V operating voltage levels of mercury oxide batteries and silver oxide batteries, respectively. Similarly, the current capacity of Ni-Cd batteries is on the order of 20 mAH, which is small in comparison with the 200 mAH and 165 mAH current capacities provided by mercury oxide batteries and silver oxide batteries, when the Ni-Cd batteries are the same conventional size (11.6 mm.phi..times.5.6 mm) as the silver oxide and mercury oxide batteries. Moreover, Ni-Cd batteries have been found to be more susceptible to leakage than mercury oxide and silver oxide batteries.
It is noted, however, that if mercury oxide or silver oxide batteries, of the type noted above, are utilized as the secondary battery in secondary battery charging arrangements of the type that have heretofore been utilized with Ni-Cd batteries, the likelihood of leakage and/or explosion occurring is greatly increased. The leakage of electrolyte from the battery case or a swelling of the battery case results from the evolution of gas when the secondary battery is overcharged. However, if a secondary battery is selected that can be charged to a satisfactory level, such as 1.8 V for use in an electronic wristwatch, and any charging thereof is inhibited when the voltage level of the secondary battery reaches a satisfactory level, no gas will be generated therein, thereby resulting in a highly effective charging of the secondary battery. Accordingly, a charge control circuit wherein the voltage level of the secondary battery is detected and utilized to control and amount of charging current produced by the charging device in order to effect charging of silver oxide, mercury oxide and also Ni-Cd batteries, is desired.
It is noted that charge control circuits that admit of a high current consumption have been proposed. For example, circuitry, wherein a pulse having a narrow pulse width of several m-sec., is utilized to detect the voltage level of the secondary battery during the short period that pulse is applied thereto. Thereafter, a pulse having a longer pulse width such as several hundred m-sec., is applied at an alternative time, so that the level of the charge control voltage is compared with voltage level of the secondary battery, with the difference therebetween stored in a memory. When the voltage level of the secondary battery is less than the charging-control voltage, a charging current is supplied to the secondary battery. When the voltage level of the secondary battery exceeds the voltage level of the charging voltage a charging current is no longer applied. However, if the secondary battery voltage is elevated above the charging voltage at the time that the respective levels are read into the memory, and the level of the secondary battery is lower than the level of the charing voltage when the shorter pulse is applied thereto, the current consumption in the secondary battery is excessive. Moreover, such circuitry is complex and requires a memory, a pulse generator and other circuit components.